feat: implement rate limiting and caching layer for MusicBrainz provider

- Add golang.org/x/time/rate dependency for token-bucket rate limiting
- Replace custom channel-based rate limiter with rate.NewLimiter(1, 1)
- Add context.Context support to doGet for cancellation
- Add cached_at column to external_releases via migration 005
- Implement cache hit/miss queries with TTL-based filtering
- Add CacheStats type for tracking cached RGIDs
- Update ExternalRelease struct with CachedAt field
- Add rate limiting tests (1 req/sec enforcement, burst behavior)
- Add cache tests (hit, miss, expired, mixed, empty artist)
- Update migration count test for new migration

internal/database/database.go

@@ -115,6 +115,10 @@ func (db *DB) migrate() error {
 				PRIMARY KEY (rgid, sent_at)
 			);`,
 		},
+		{
+			name: "005_add_cached_at_to_external_releases",
+			sql:  `ALTER TABLE external_releases ADD COLUMN cached_at DATETIME;`,
+		},
 	}
 
 	for _, m := range migrations {
@@ -177,12 +181,13 @@ type LocalAlbum struct {
 
 // ExternalRelease represents a row in the external_releases table.
 type ExternalRelease struct {
-	RGID        string `json:"rgid"`
-	ArtistID    string `json:"artist_id"`
-	Title       string `json:"title"`
-	Type        string `json:"type"`
-	ReleaseDate string `json:"release_date"`
-	IsIgnored   bool   `json:"is_ignored"`
+	RGID        string    `json:"rgid"`
+	ArtistID    string    `json:"artist_id"`
+	Title       string    `json:"title"`
+	Type        string    `json:"type"`
+	ReleaseDate string    `json:"release_date"`
+	IsIgnored   bool      `json:"is_ignored"`
+	CachedAt    time.Time `json:"cached_at"`
 }
 
 // NotificationSent represents a row in the notifications_sent table.